Pulmonary hypertension (PH) is a frequently lethal condition with no known cure. Current therapies primarily target vasodilation, but for a subset of patients, pulmonary inflammation may also be a pathogenic factor. Lungs from animals and patients with severe PH exhibit increased perivascular CD68+ macrophages; these cells when removed from the lungs of rats with PH induce pulmonary artery endothelial cell (PAEC) apoptosis in culture. Leukotriene B4 (LTB4) secreted by these lung macrophages appears to be the key mediator responsible for vascular injury in this animal model. LTB4 also generates apoptosis-resistant endothelial cells in vitro while promoting pulmonary artery smooth muscle cell (PASMC) and fibroblast growth. LTB4 is elevated in the lungs and blood of rats and patients with PH, and antagonizing LTB4 reverses severe experimental PH. These cumulative findings suggest that LTB4 antagonism holds promise as a new adjunctive therapy for patients suffering this deadly disease. Studies proposed in this grant address how macrophage- derived LTB4 specifically affects the three blood vessel layers of pulmonary arterioles (PAECs, PASMCs, adventitial fibroblasts) as well as the correlation between LTB4 production and the clinical presentation of PH. The proposal's general hypothesis is that during the evolution of PH, macrophage-derived LTB4 kills healthy native PAECs by inhibiting key survival signals while also promoting apoptosis-resistance in surviving, phenotypically-altered ECs, and that LTB4 enhances the proliferation of PASMCs and fibroblasts via tissue- specific BLT1-signaling pathways. Specific Aim 1 is to determine how macrophage-derived LTB4 mediates PH- relevant PAEC injury through its impact on bone morphogenetic protein receptor 2 (BMPR2) and endothelial nitric oxide synthase (eNOS) survival signaling. Aim 2 is to determine whether macrophage-derived LTB4 generates apoptosis-resistant PAECs with molecular stemness signatures. Aim 3 is to determine the role of LTB4 in PASMC and fibroblast proliferation. Finally, Aim 4 is to determine plasma LTB4 levels and lung LTB4 biosynthesis in PH patients and correlate these findings with clinical activity.